The present invention relates to the use of chemically stable solid ion conductors having a garnet-like structure in batteries, supercapacitors, accumulators and electrochromic devices, chemical sensors and thermoelectric converters, and also novel compounds which are suitable for these uses.
Rechargeable (secondary) batteries are used where grid-independent operation of electric and electronic appliances is necessary or desired for at least part of the time. Research on solid ion conductors as electrolyte materials for this use forms, in this context, an important aspect of current material research. The advantages sought in a battery composed only of solids are guaranteed freedom from leaks, miniaturizability, electrochemical stability, relatively high energy densities and a relatively long life.
Among the various battery technologies, battery systems based on lithium ions have become increasingly established in recent years. They are particularly notable for their high achievable electric energy density and power, which are attributable to the high chemical reactivity and the low mass of lithium ions and also their high mobility. The development of solid lithium ion conductors has attracted considerable attention in recent years. Examples are Li2.9PO3.3N0.46 or Li3N and Li-β-aluminium oxide. However, Li2.9PO3.3N0.46 has a significantly lower ion conductivity than liquid electrolytes. Li3N and Li-β-aluminium oxide are very sensitive to moisture. In addition, Li3N decomposes at a voltage as low as 0.445 V at room temperature and Li-β-aluminium oxide is not chemically stable.
Lithium ion conductors having a garnet-like structure were described for the first time in the study by Thangadurai et al., “Novel Fast Lithium Ion Conduction in Garnet-Type Li5La3M2O12 (M=Nb, Ta)”, J. Am. Ceram. Soc. 86, 437-440, 2003. The garnet-like Li5La3M2O12 compounds have an appreciable lithium ion conductivity.
In structural terms, garnets are orthosilicates of the general composition X3Y2(SiO4)3 which crystallize in the cubic crystal system, where X and Y are octacoordinated and hexacoordinated cation sites. The individual SiO4 tetrahedra are connected to one another by ionic bonds via the interstitial B cations.
The garnet-like compounds of the formula Li5La3M2O12 (M=Nb, Ta) which are described in the above-mentioned study by Thangadurai et al. contain an excess of Li ions compared to an ideal garnet structure. The La3+ and M5+ ions occupy the octacoordinated and hexacoordinated sites, while lithium ions occupy positions having six-fold coordination.
The PCT application WO 2005/085138 reports that further garnet-like lithium ion conductors are obtained formally by aliovalent substitution from the compounds of the formula Li5La3M2O12 (where M=Nb or Ta). Aliovalent substitution of the La3+ sites can increase the connectivity of the network and enables the number of available vacancies to be varied. Charge balance is preferably achieved by means of Li+ ions (L). For the purposes of the present invention, “aliovalent substitution” means the replacement of an ion by an ion having a different oxidation state, as a result of which cation vacancies, anion vacancies, interstitial cations and/or interstitial anions are formed. The solid lithium ion conductors are chemically stable and have an ion conductivity of more than 3.4×10−6 S/cm. Owing to their high ion conductivity accompanied by negligible electron conductivity, they can be used as solid-state electrolytes.
The compounds described in WO 2005/085138 generally have the stoichiometric composition L5+xAyGzM2O12, where    L is in each case independently any preferred monovalent cation,    A is in each case independently a monovalent, divalent, trivalent or tetravalent cation,    G is in each case independently a monovalent, divalent, trivalent or tetravalent cation,    M is in each case independently a trivalent, tetravalent or pentavalent cation,    0<x≦3, 0≦y≦3, 0≦z≦3 and    O can be partly or completely replaced by divalent and/or trivalent anions such as N3−.
In the ion conductors described M is in each case one of the metals Nb and Ta. Other examples of metal ions are not given. Ion conduction occurs via lithium ions (L=Li).
Further examples of lithium ion conductors having a garnet structure have been examined in recent years (V. Thangadurai, W. Weppner, Adv. Funct. Mater. 2005, 15, 107-112; V. Thangadurai, W. Weppner, J. Power Sources, 2005, 142, 339-344). Here, Li6BaLa2Ta2O12 had the highest Li+ ion conductivity of 4×10−5 Scm−1 at 22° C. with an activation energy of 0.40 eV. While Li6BaLa2Ta2O12 is stable towards reaction with metallic lithium, moisture, air and customary electrode materials, the volume conductivity and total conductivity at room temperature are still not sufficiently high to enable an ideal rechargeable solid lithium ion battery to be developed.
Another problem associated with the above ion conductors of the prior art is that the proposed metals niobium and tantalum are relatively expensive and not readily available. In addition, the use of a solid electrolyte which consists entirely of the garnet-like compounds described is complicated and associated with high costs.